The present invention relates to a semiconductor laser device having a protective coating with a specified reflectance formed on light emitting end surface, and to a method for manufacturing the same.
As shown in FIG. 5, most semiconductor laser devices are composed of, for example, protective coatings 2a and 2b, each having an identical reflectance, formed on light emitting end surfaces 1a and 1b of a GaAs laser chip 1. Reference numeral 3 denotes an active layer of the laser chip 1. In the case where the protective coatings 2a and 2b are composed of Al2O3 in FIG. 5, if a refractive index of the Al2O3 film is set to 1.60 while a refractive index of the laser chip 1 is set to 3.50, a reflectance of the protective coatings 2a and 2b corresponding to a coating thickness d varies as shown in FIG. 6 (provided that a laser emission wavelength xcex=7800 xc3x85).
FIG. 6 indicates that regardless of the coating thickness d of the protective coatings 2a and 2b, the reflectance thereof is smaller than that of the case without the protective coatings 2a and 2b (i.e. the reflectance of the light emitting end surfaces 1a and 1b). The reflectance becomes smallest when an optical coating thickness (refractive index nxc3x97coating thickness d) is an odd multiple of xcex/4, while the reflectance becomes approximately equal to that in the case without the protective coatings 2a and 2b when the optical coating thickness is an integral multiple of xcex/2. This is because the refractive index (1.60) of the protective coatings 2a and 2b is smaller than the refractive index (3.50) of the GaAs laser chip 1.
Contrary to this, in the case where the refractive index of the protective coatings 2a and 2b is larger than the refractive index of the GaAs laser chip 1 (for example, if such material as Si is used as the protective coatings 2a and 2b, the reflectance thereof is larger than that in the case without the protective coatings 2a and 2b, regardless of the coating thickness), the reflectance becomes largest when the optical coating thickness is an odd multiple of xcex/4, while the reflectance becomes approximately equal to that in the case without the protective coatings 2a and 2b when the optical coating thickness is an integral multiple of xcex/2.
In the case of high output semiconductor laser devices with optical output as high as 20 mW or more, as shown in FIG. 7, for increasing optical output Pf from the side of a main emitting end surface (front end surface), the reflectance of the protective coating 12a on the side of the main emitting end surface 11a is generally set lower than that in the case without the protective coating 12a, while the reflectance of the protective coating 12b on the side of a rear emitting end surface 11b is set higher than that in the case without the protective coating 12b. For example, the reflectance of the protective coating (Al2O3) 12a is set to approx. 15% or less. This reflectance is obtained with the coating thickness of approx. 700 xc3x85 to 1600 xc3x85.
The protective coating 12b on the rear emitting end surface 11b, if composed with use of a film having a refractive index larger than that of the laser chip 11, is not capable of providing a sufficiently high reflectance as a single layer. Accordingly, there are laminated an Al2O3 film with a thickness of xcex/4 as a first layer 14 and a third layer 16, and an amorphous Si with a thickness of xcex/4 as a second layer 15 and a fourth layer 17. Then finally, there is laminated an Al2O3 film with a thickness of xcex/2 as a fifth layer 18. This makes it possible to form a protective coating 12b having a reflectance as high as approx. 85% or more. It is noted that reference numeral 13 denotes an active layer.
Description will now be given of a method for forming protective coatings 2a and 2b having the above-described reflectance on light emitting end surfaces 1a and 1b of a semiconductor laser chip 1.
First, as shown in FIG. 8, there is formed by scribing a cleavage line 25 extensively disposed between an electrode 22 of an arbitrary element in a semiconductor laser wafer 21 and an electrode 23 of an adjacent element in direction orthogonal to an emitting section (channel) 24. Then, as shown in FIG. 9, the semiconductor laser wafer 21 is cleaved and divided into a plurality of laser bars (bar-shaped laser chips) 26.
Next, as shown in FIG. 10, a plurality of the divided laser bars 26 are set in a laser bar fixing device 27 such that the electrodes 22 are laid on top of each other. All the laser bars 26 should be set so that an emitting end surface 28a and an emitting end surface 28b face the same side. Next, on the emitting end surfaces 28a and 28b of a laser bar 26 fixed to the laser bar fixing device 27, there is formed a protective coating having a specified reflectance, generally with use of a vacuum evaporator 29 exemplarily shown in FIG. 11. The vacuum evaporator 29 is provided with a vapor source 31, a holder 32 for holding a plurality of the laser bar fixing devices 27, and a crystal oscillator 33 for monitoring the thickness of evaporated films, all in a chamber 30.
Following description discusses procedures of forming the protective coating. First, in the case for evaporating a protective coating onto the emitting end surface 28a, the holder 32 is disposed such that the emitting end surface 28a of a laser bar 26 faces the vapor source 31 side as shown in FIG. 11. Then, the chamber 30 is evacuated through a duct 34. After a specified degree of vacuum is obtained, an evaporation material 35 put in the vapor source 31 is heated and evaporated by electron beams and the like so that a protective coating is evaporated onto the emitting end surface 28a of the laser. After evaporation is completed, the holder 32 is then rotated 180xc2x0 for evaporating a protective coating onto the emitting end surface 28b based on the same procedures.
Here, a forming speed (evaporation rate) for forming a protective coating on the both light emitting end surfaces 28a and 28b is controlled to be approximately constant till completion of evaporation. The evaporation rate is in this case controlled with heating temperature. In the case of electron beam evaporation, therefore, the evaporation rate may be controlled with intensity of electron beams. It is well known that in the case of resistance heating, the evaporation rate is controlled with an amount of electric current passed through a resistance. The evaporation rate is generally set to the range between several xc3x85/sec to 30 xc3x85/sec with the evaporation material of Al2O3. Evaporation is conducted while coating thickness is monitored with use of the crystal oscillator 33. Evaporation is terminated when a specified coating thickness is obtained.
In the case of a high output type semiconductor laser device shown in FIG. 7, there is formed a low reflecting protective coating 12a (having a reflectance of approx. 15% or less) on the side of the main emitting end surface 11a, and then there is formed in succession a multilayered high reflecting protective coating 12b on the side of the rear emitting end surface 11b. The multilayered high reflecting protective coating 12b is composed of a laminated structure made up of: a first layer 14 and a third layer 16 each consisting of an Al2O3 film with a thickness equal to xcex/4; a second layer 15 and a fourth layer 17 each consisting of an Si film with a thickness equal to xcex/4; and a fifth layer 18 consisting of an Al2O3 film with a thickness equal to xcex/2. For evaporation of this film, Al2O3 and Si are mounted on the vapor source 31 as evaporation materials 35. Then the first layer 14, the third layer 16, and the fifth layer 18 consisting of an Al2O3 film are evaporated through irradiation of the evaporation material Al2O3 with electron beams, and the second layer 15 and the fourth layer 17 consisting of an Si film is evaporated through irradiation of the evaporation material Si with electron beams.
For high input type semiconductor laser devices, as shown in FIG. 12, there has been proposed a method for forming a protective coating 42a on the side of a main emitting end surface 41a of a laser chip 41 utilizing high thermal conductivity of Si, in which an Si film 44 having high thermal conductivity is formed first and then a low reflecting protective coating 45 is formed (Japanese Patent Laid-Open Publication HEI No. 1-289289). In the drawing, reference numeral 42b denotes a multilayered high reflecting protective coating on the side of a rear emitting end surface 41b composed of a first layer 46, a second layer 47, a third layer 48, a fourth layer 49, and a fifth layer 50, and reference numeral 43 denotes an active layer.
In this example, heat generated in the vicinity of the main emitting end surface 41a by light emission of the semiconductor laser device is efficiently discharged by the Si film 44, which controls deterioration of the semiconductor laser device caused by long term supply of current. The Si film has a film thickness of around xcex/4 (approx. 532 xc3x85 in an embodiment).
However, the above-stated background art semiconductor laser devices have a following problem. In forming protective coatings 2a, 2b, 12a, and 12b of laser chips 1 and 11 by evaporation, an oxide (Al2O3), that is a material of the protective coatings 2a, 2b, 12a, and 12b, is decomposed to generate oxygen immediately after start of evaporation process, which increases partial pressure of oxygen molecules. The oxygen, colliding or bonding with end surfaces 1a, 1b, 11a, and 11b of the laser chips 1 and 11, is highly likely to cause damage to the end surfaces 1a, 1b, 11a, and 11b. Further, in the case where active layers 3 and 13 of the laser chips 1 and 11 and vicinity layers thereof contain aluminum, the damage is considered to be larger. If thus-fabricated semiconductor laser device is operated with high output, necessary reliability may not be provided.
Further, according to the high output type semiconductor laser device disclosed in the Japanese Patent Laid-Open Publication HEI No. 1-289289, in forming protective coating 42a on the main emitting end surface 41a, the Si film 44 having high thermal conductivity is formed first for increasing reliability. In this case, there is formed first the Si film 44, which is free from generation of oxygen due to decomposition of the material in the process of evaporation, thereby enabling creation of a coating in the vicinity of the emitting end surface 41a of the laser chip 41 immediately after start of evaporation process under conditions of low partial pressure of oxygen. Therefore, in addition to increase of heat dissipation, there may be achieved an effect of controlling the above-stated damage in the vicinity of the emitting end surface 41a. 
In this case, however, the Si film 44 has a thickness as high as approx. 532 xc3x85 (almost equal to xcex/4), which may cause leakage current in the Si film 44 (light emitting end surface), and may affect oscillation characteristics of the semiconductor laser device.
Accordingly, it is an object of the present invention to provide a semiconductor laser device capable of reducing damages given to a light emitting end surface in creation of a protective coating, and of controlling generation of leakage current in the vicinity of the light emitting end surface, as well as to provide a method for manufacturing the same.
In order to achieve the above object, there is provided a semiconductor laser device, comprising: an oxide having a specified reflectance formed as a protective coating on light emitting end surfaces of a semiconductor laser chip; and
an Si film having a film thickness of 40 xc3x85 or less formed between at least one light emitting end surface 51a and the oxide.
According to the above constitution, before an oxide is formed as a protective coating, there is formed an Si film, which is free from generation of oxygen due to decomposition. Consequently, creation of the coating is conducted immediately after start of Si film forming under conditions of low partial pressure of oxygen, which prevents oxygen with high energy from colliding or boding with the light emitting end surface. Further, if oxygen is decomposed in the process of oxide forming and so the oxygen partial pressure increases, collision or bonding of the oxygen with the light emitting end surface is prevented. Thus, the damages given to the light emitting end surface in the process of protective coating formation are controlled.
Here, if the semiconductor laser chip has an active layer including Al, the damages given to the light emitting end surface is effectively controlled.
In addition, the Si film has a film thickness as small as 40 xc3x85 or less. This reduces generation of leakage current in the Si film or on the light emitting end surface, thereby preventing negative influence on the oscillation characteristics.
In one embodiment of the present invention, the Si film has a film thickness of 5 xc3x85 or more and 30 xc3x85 or less.
According to the above constitution, the Si film has a film thickness of 5 xc3x85 or more and 30 xc3x85 or less, which almost eliminates generation of the leakage current.
In one embodiment of the present invention, the oxide constituting the protective coating is an Al2O3 film.
According to the above constitution, if the semiconductor laser chip is formed with GaAs, the refractive index of the oxide as the protective coating is smaller than the refractive index of the semiconductor laser chip, and therefore the reflectance of the protective coating is smaller than the reflectance of the light emitting end surface, regardless of the coating thickness. This increases optical output from the light emitting end surface.
In one embodiment of the present invention, the Si film has purity of 99.99% or more.
According to the above constitution, the Si film has purity of 99.99% or more. This ensures more effective prevention of oxygen with high energy from colliding or boding with the light emitting end surface.
Also, there is provided a method for manufacturing the semiconductor laser device, comprising
a step of forming the Si film and the oxide on the light emitting end surface, the step being executed in succession within same equipment without exposing the surface to the air.
According to the above constitution, there is formed a semiconductor laser device, which decreases damages on the light emitting end surface and reduces generation of leakage current in the Si film or on the light emitting end surface, based on almost the same process as the background art.
Also, there is provided a method for manufacturing the semiconductor laser device, comprising
a step of forming the Si film and the oxide through vacuum deposition.